Right, Schumer is blowing smoke.
And then, if the vote was to allow a witness called by the D's, Trump could, subject to his trial plan, assert executive privilege and take the matter through the federal court system to the USSC.
The legal issue in one form would be under what circumstances should a president's right of executive privilege be outweighed by a political fishing expedition (there being no underlying criminal offense).
The process would likely consume a period of several weeks. It would be entertaining to see the D's and the MSM's spin during that period.
The problem is that they might do it. The Ds really want to drag this out as long as possible.